For certain technical applications, it is necessary to reduce the carbon content of steel to a low value. An important example is the removal of carbon from thin sheet steel laminations used in the magnetic circuits of electric motors and transformers. It is desired in this application to lower the carbon level to a few thousandths of a percent in order to minimize hysteresis losses. A further objective, usually achieved as a part of the same process by which the carbon content of the laminations is lowered, is the production of a thin, adherent coating of iron oxide on the lamination. This oxide coating, having a low electrical conductance, effectively insulates the laminations from one another and prevents the flow of eddy currents which would result in large electical losses. The oxide coating appears as a uniform dark blue coloration on the surface of the finished part.
The decarburization is normally carried out below the ferrite-austenite transition temperature of pure iron, that is, below a temperature of about 1670.degree. F. (910.degree. C.). A typical decarburization temperature is 1450.degree. F. (788.degree. C.) although higher or lower temperatures may be employed if desired. By carrying out the decarburization below the transition temperature, as carbon is removed from the piece large crystals of alpha iron (ferrite) grow from the surface inward. This large-grained structure confers good magnetic properties to the finished parts.
Decarburization is achieved by exposing the parts to an atmosphere having a composition such that carbon dissolved in the metal reacts to produce gaseous products which are then swept away from the surface. Conventional heat treating literature alleges that three substances normally found in heat treating atmospheres are capable of reacting with dissolved carbon to produce gaseous products. These are hydrogen, water and carbon dioxide, according to the following reactions: EQU C+2H.sub.2 .fwdarw.CH.sub.4 ( 1) EQU C+H.sub.2 O.fwdarw.CO+H.sub.2 ( 2) EQU C+CO.sub.2 .fwdarw.2CO (3)
Water is regarded as being effective at low concentrations, with carbon dioxide acting much slower and hydrogen as having very little reactivity. Water and carbon dioxide, if present in sufficiently high concentration, are capable of oxidizing the iron to iron oxides according to the following equations: EQU Fe+xH.sub.2 O.fwdarw.FeO.sub.x +xH.sub.2 ( 4) EQU Fe+xCO.sub.2 .fwdarw.FeO.sub.x +xCO (5)
Where x ranges from 1.0 to 1.5
Water is much more potent than carbon dioxide as an oxidizing agent. Although a final oxidation to produce the thin, adherent insulating coat is desired, oxidation must be avoided during the decarburization process so that the decarburization agent has free access to the metal surface, and outward diffusion of carbon is not hindered by an oxide layer.
Further it is important that oxide formation shall not occur at temperatures above 1030.degree. F. (554.degree. C.), since ferrous oxide, FeO, will be formed, whereas below this temperature magnetite, Fe.sub.3 O.sub.4, is produced. Ferrous oxide may cause laminations, which are commonly decarburized in stacks, to adhere to one another while magnetite is much less objectionable.
Traditionally, decarburizing atmospheres have been generated in a number of ways. One process involves the production of so-called exothermic gas by combustion of natural gas in air. The resulting atmosphere consists of nitrogen, carbon dioxide, water, and, depending upon the ratio of fuel to air, more or less hydrogen and carbon monoxide. It may be necessary to cool the gas to condense part of the large amount of water and then reheat it in order to avoid oxidation of the metal. The rising cost of natural gas, its short supply and its variable composition make it an increasingly less attractive primary source for generating an atmosphere.
Another atmosphere which has been employed is a humidified hydrogen/nitrogen mixture such as disclosed in U.S. Pat. No. 3,098,776. A three-to-one hydrogen/nitrogen mixture may be produced by the cracking of ammonia. An alternate approach is to employ relatively low-cost nitrogen to which is added a small quantity of hydrogen. To both of these atmospheres it is necessary to add water, either as steam or as a liquid which is then vaporized. The advantage of this approach is the consistent composition of the atmosphere and simpler process equipment. The disadvantage is that it is essential that the concentration of water in the atmosphere be carefully controlled to a low level to avoid the possibility of early oxidation of the metal surface. Another disadvantage is that the hydrogen/nitrogen atmosphere may cost more than an exo-atmosphere.
Another process is disclosed in U.S. Pat. No. 4,285,742 wherein mixtures of an inert gas, water and a compound of carbon, hydrogen and oxygen are used to effect decarburization of electrical steels. The compounds of carbon, hydrogen and oxygen identified by patentees are preferably methanol with additions of, or alternatively, a high aliphatic alcohol and/or acetone. The composition is selected so that the furnace atmosphere, at temperature, contains at least 1% water vapor.